W3C markup validation - The source to go to check HTML, XHTML or other markup language you use on your web page for errors.
Google Webmaster Tools - A must for any Webmaster. With this free service, you can identify any page or links with errors, scan for malware, find pages with short or missing titles, find duplicate meta tags, and much more. You need a Google account to use these tools.
Silktide Nibbler - Fantastic free online service that gives you a good complete overview of your website with an overall score.
Woorank - Another fantastic free online tool that gives you a quick and easy to read overview of problems you may have on your site.
HTML Tidy - Simple and easy to use online tool that allows you to identify any HTML errors you may have on your page.
Feng-GUI - Great service that allows you to upload a picture of your home page or other pages and generate a heat map of where the visitor's eyes are most likely to go first.